Project Summary Countries around the world are currently experiencing the long-term effects of prior demographic changes. Historical drops in mortality and fertility rates associated with the first demographic transition are causing dramatic population aging in many societies, with more to follow. More recently, the second demographic transition engendered a reorganization of family formation processes in many places; these processes are ongoing, but retreats from marriage, increased divorce, and greater levels of childlessness are spreading to ever more contexts. The natural consequence of these combined forces is a world where many countries have increasingly larger subpopulations of older adults lacking the types of living family members that are the mainstays of social support networks. In this project, we aim to examine how the family networks of older adults are changing across countries, how they will likely change in the future, and how shifts in kin availability will impact health through mechanisms of loneliness and social isolation. In the proposed project, we will undertake these goals by combining data from numerous nationally representative studies with computational social network and demographic projection methods to examine the current and rapidly growing future prevalence of older adults who do not have any living family members ? what we refer to as kinlessness ? and we will test what these trends imply for population health now and in the future. Using 17 different data sets from 36 countries, our estimates of family structure and health provide coverage for 70.5% of the global population of adults ages 45 and above, with some representation in each world region. Using these data, we will first document national differences in prevalence of kinless older adults according to several definitions of kinlessness (e.g. lacking a spouse/partner and children; having none of the following kin types: spouse/partner, children, parents, or siblings; etc.). Next, we will examine associations between kinlessness and health, with specific attention to loneliness and social isolation mechanisms, including examining kin geographic proximity, and contact, communication, transfers, and exchanges with kin and non-kin. After this, we will use social network simulation and demographic projection methods to situate contemporary estimates of kinlessness in historical context for each country in our study. Doing so will allow us to provide clear estimates of the future trajectory of trends in kinlessness in different contexts and to characterize general processes and contextual variation in its likely unfolding. As part of this endeavor, we will apply estimates of relationships between lacking living kin and health to our projections in order to characterize how kinlessness and family structure trends might affect global health in the coming decades. We will also test sensitivities to uncertainty about future demographic trends and alternative scenarios. This project will tie together how ongoing changes in demography and families over the long run are shifting the number of kin available to older adults, address how the kinless population fares across contexts in terms of health and social isolation, and provide the first characterization of the implications for population health of global variability and changes in older adult family structures now and in the future.